Basic Plasma Parameters

Consider an idealized plasma consisting of an equal number of electrons, with mass me and charge -e (here, e denotes the magnitude of the electron charge), and ions, with mass mi and charge +e. We do not necessarily demand that the system has attained thermal equilibrium, but nevertheless use the symbol to denote a kinetic temperature measured in energy units (i.e., joules). Here, v is a particle speed, and the angular brackets denote an ensemble average.

The kinetic temperature of species S is essentially the average kinetic energy of particles of this species. In plasma physics, kinetic temperature is invariably measured in electron-volts (1 joule is equivalent to 6.24×1018 eV).

Quasi-neutrality demands that

where ns is the number density (i.e., the number of particles per cubic meter) of species S.

Assuming that both ions and electrons are characterized by the same T (which is, by no means, always the case in plasmas), we can estimate typical particle speeds via the so-called thermal speed.

Note that the ion thermal speed is usually far smaller than the electron thermal speed: